The present invention relates to catalytic reactors for carrying out dehydrogenation reactions, and has particular application to reactors for the dehydrogenation of ethylbenzene to produce styrene.
A well-known process for the commercial production of styrene via the dehydrogenation of ethylbenzene involves combining ethylbenzene with steam to form a diluted feed stream which is then further heated to a suitable reaction temperature (e.g., 500-700xc2x0 C.) in a preheating furnace. The pre-heated feed stream is then fed at low pressure into a catalytic reactor typically containing a bed of iron oxide-based catalyst pellets. The dehydrogenation reaction to produce styrene is endothermic, with styrene yields being favored at relatively high temperatures and low pressures.
The complex product of the dehydrogenation reaction generally comprises a vaporized mixture of styrene and unreacted ethylbenzene together with water vapor, H2, CO, CO2, light hydrocarbons, benzene, toluene, and heavier components such as polymerized by-products. The vapor mixture is cooled to condense and separate the liquid phase from the gas phase, and the separated liquid is processed through a series of distillation columns to recover the styrene and recycle the unreacted ethylbenzene.
Commercial styrene production typically involves processing the ethylbenzene through two or more catalytic reactor stages operating in series, with the feed stream being reheated between stages to compensate for the heat lost during the endothermic process. U.S. Pat. No. 4,347,396 discloses a processing system incorporating a series of reactors for this purpose.
Since the equilibrium constant for ethylbenzene-to-styrene conversion depends inversely on reactor pressure, pressure drops within commercial reactors are kept as low as possible. A high catalyst surface area:volume ratio increases mass transfer efficiency and suppresses undesirable by-product formation. Catalyst utilization and efficiency can be improved somewhat by using tightly packed beds of relatively small catalyst beads or pellets, but such use increases reactor pressure drop, tending to negate the expected improvements in catalytic conversion rate and styrene selectivity.
One way to address the pressure drop problem is to deploy the packed pellet beds in a radial flow reactor. One example of such a reactor is disclosed in U.S. Pat. No. 5,358,698. Compared to the pressure drops developed by unidirectional feed stream flows across conventional packed catalyst beds, radial flow reactors significantly reduce pressure drop by dispersing the catalyst beads around a large cylindrical volume encircling the feed stream inlet. At the same time, the gas linear velocity through the distributed pellet bed is reduced.
Unfortunately, however, several drawbacks associated with radial flow reactors remain. Among these are the fact that the gas distribution and collection chambers for these reactors take up a large fraction (30xcx9c60%) of the overall volume of the reactor. Thus, the space utilization efficiency of radial reactors is low.
In addition, radial reactor designs require that the feed stream change flow direction at least twice between the gas inlet and gas outlet of the reactor. This kind of flow pattern can cause poor gas distribution over the packed bed along the reactor axis that can reduce catalyst efficiency, and can increase catalyst attrition along the edges of the cylindrical beds. Extra catalyst bed support structures and screening are also required, increasing the capital cost of the reactor and introducing operating reliability issues.
Finally, it is more difficult and expensive to introduce heat uniformly into the middle of the catalyst bed in these reactors. Thus larger temperature gradients tend to develop within the catalyst bed that negatively affect conversion, selectivity, and catalyst stability.
Honeycomb monolithic catalysts such as disclosed in U.S. Pat. No. 4,711,930 have been considered for use in dehydrogenation reactions but have found little use in commercial chemical processing systems. In principle, these catalysts should offer reduced feed stream pressure drops and improved heat/mass transfer efficiency when compared with pelletized catalysts. However, the art has not yet developed dehydrogenation reactor designs that effectively exploit the performance characteristics of these catalysts.
The present invention provides axial flow reactor designs offering efficiencies higher than those of existing pellet bed and radial flow reactors, while at the same time maintaining or reducing reactor size and capital cost. The designs can be employed in standalone systems, or they can be used for retrofit or supplemental reactors that can significantly improve the efficiency of existing series reactor systems.
The improved reactor designs of the invention include an improved axial flow dehydrogenation reactor that effectively exploits the advantages of honeycomb catalyst packing (hereinafter also referred to as monolithic packing). The reactor assembly includes a reaction chamber having an inlet and an outlet and containing two or more beds of monolithic catalyst disposed therewithin. The catalyst beds include an upstream bed and a downstream bed disposed in series along the reactant flowpath, the latter generally following a flow axis traversing the chamber from an upstream to a downstream direction between the chamber inlet and outlet.
Each of the catalyst beds in the chamber is formed of one or more monolithic dehydrogenation catalysts, each of which is a honeycomb catalyst incorporating a plurality of open-ended honeycomb channels traversing the catalyst from the upstream to the downstream direction on the reactant flowpath. These channels provide catalytically active channel wall surfaces for treating a heated vapor stream containing a hydrogen-containing reactant passing through the catalyst bed, at least partially converting the reactant to a dehydrogenated product in an efficient manner and at low pressure drops.
Also disposed within the chamber, and situated between the upstream and downstream catalyst beds to separate them from one another, are heating means for re-heating the vapor stream after traversal of the upstream catalyst bed. The heating means, which are preferably also designed to operate a low pressure drop, provide an effective and space-efficient way to restore heat energy to the reactant stream prior to its traversal of the downstream reactor bed.
Reactors of the described type may be utilized alone or in series to carry out a variety of endothermic dehydrogenation reactions at conversion rates equivalent or better than achievable in packed bed reactor systems. However, an alternative and preferred use of such a reactor is in combination with a radial flow reactor in a multiple-stage dehydrogenation reactor systems for the conversion of ethylbenzene to styrene. By a multiple-stage reactor system is meant a reactor system incorporating two or more dehydrogenation reactors in series, each reactor constituting a stage in the dehydrogenation process.
In another aspect the invention includes a multiple-stage dehydrogenation reactor system comprising at least two reactors in series. The system includes an axial flow reactor stage connected with a second reactor stage, the second stage being a second axial flow reactor stage or, more typically, a radial flow reactor stage.
The axial-flow reactor stage comprises a first reaction chamber having a first inlet and a first outlet and containing at least one monolithic catalyst bed of the kind above described, i.e., incorporating one or more monolithic honeycomb catalysts, each of which has honeycomb channels disposed along the flow axis within the chamber. More preferably, the axial flow stage will include at least two monolithic catalyst beds separated by heating means as above described, the upstream bed and downstream bed again being disposed in series along the flow axis and the heating means operating to add heat to reactant stream under endothermic reaction conditions.
Where a radial flow reactor stage is used as the second stage in the dehydrogenation reactor system, the radial stage may be of conventional design. Typically, it will include a second reaction chamber having a second inlet and a second outlet wherein the inlet is connected directly or indirectly to the outlet of the first reaction chamber or axial flow reactor stage.
Included within the second reaction chamber in the case of a radial flow reactor are a central inlet section for collecting the reactant stream entering the chamber through the inlet, a radial flow catalyst bed distributed about and encircling the inlet section for treating the reactant stream collecting in the inlet section, and a surrounding outlet section for collecting the product stream produced by passage of the reactant stream through the catalyst bed.
As in most radial reactor designs, the outlet section is a peripheral or circumferential volume occupying the space between the catalyst bed and the chamber wall, within which section the product stream may be collected as it flows radially outwardly through the distributed catalyst bed. The outlet section connects with the chamber outlet from which the collected product stream is discharged from the reactor.
Whether or not heating means are provided within the axial flow reactor stage in such a reactor, most two-stage reactor designs will include some inter-stage heating means for adding heat to the reactant stream. Such heating means will be disposed downstream of the monolithic catalyst bed(s) present in the axial reactor stage and upstream of the radial catalyst bed. In case of lack of space in the radial flow reactor chamber, such heating means may be positioned between the outlet of the axial stage and the inlet of radial stage, More typically, however, it is incorporated as part of the radial flow reactor at the inlet end thereof. In either case the heating means will be sufficient in capacity to reheat the cooled reactant stream issuing from the outlet of the axial stage prior to delivering it to the catalyst bed within the radial flow reactor.
The invention further comprises a method for treating a reactant stream to at least partially dehydrogenate a hydrogen-containing reactant present therein. In accordance with that method the reactant stream is heated to a first dehydrogenation reaction temperature and is then conveyed through a first dehydrogenation reactor containing at least one catalyst bed of monolithic honeycomb dehydrogenation catalysts. This step of the method effects dehydrogenation of at least a portion of the reactant to produce a partially dehydrogenated intermediate stream.
Thereafter the intermediate stream is heated to a second dehydrogenation reaction temperature and is conveyed through a second dehydrogenation reactor containing at least one bed of a second dehydrogenation catalyst. This step effects a further dehydrogenation of the intermediate stream to produce a dehydrogenated product stream.
The second dehydrogenation catalyst can be a monolithic dehydrogenation catalyst of honeycomb shape, or it can be another low-pressure-drop dehydrogenation catalyst bed. In the latter case the low pressure drop catalyst bed will typically be a short path length bed made up of a granular, beaded, or pelletized dehydrogenation catalyst, such as the shallow catalyst bed of a radial flow dehydrogenation reactor.
The enhanced reactor productivity and conversion efficiency of the monolithic catalyst reactors of the present invention improve dehydrogenation performance in a number of ways. For example, the difficulties of separating dehydrogenated product from feed, including the separations required in the ethylbenzene/styrene conversion process, are well known. The reactors of the invention alleviate these difficulties through higher one-pass dehydrogenation conversions at the same or higher capacities, resulting in substantial savings in downstream separation costs and the costs of recycling unreacted.
Another benefit of using monolithic reactors is that the dehydrogenation process can be conducted under less severe reaction conditions, e.g., at reduced temperatures. Such conditions prolong catalyst lifetime and further reduces operating costs.